Recovery of mercaptans



Nov. 19, 1940. w. A. CRAIG r-:rAL

RECOVERY OF MERCAAPTANS Filed April 20. 1938 Euh. SS 385% ATTORNEYS Patented Nov. 19, 1940 UNITED STATES PATENT OFFICE RECOVERY F MERCAPTANS Application April 20,

Claims.

This invention relates to the separation of ndividual mercaptans from hydrocarbon solutions containing mixtures of several mercaptans. More particularly this invention pertains to the selec- 5 tive separation in relatively pure form of mercaptans, such as for example, ethyl mercaptan, isopropyl mercaptan, iso-butyl mercaptan and the like, from hydrocarbons which contain mercaptan mixtures.

Hydrocarbon oils, such as those produced by cracking sulfur bearing petroleum oils, contain mercaptans in mixtures of varying proportions. Such mercaptan-containing petroleum products are designated herein by the commonly employed term sour oils. Raw cracked gasoline which is produced in a conventional cracking plant from selected charging stocks may contain as much as 100 mois, more or less, of mercaptans per 1,000 liters of raW gasoline. Generally speaking, the mercaptans are classed as undesirable impurities in petroleum products, and a number of methods have been proposed to remove them` as a group from the hydrocarbon oils or to converttheminto less objectionable sulfur compounds which areleit in the hydrocarbon oils. It has beenproposed,for example, to remove mercaptans as a group from hydrocarbon oils, such as raw cracked gasolinaby Washing the hydrocarbon oils with an aqueous solution of an alkali. The eiectiveness of the removal of these undesirable mercaptans by such a Washing treatment is determined in large measure by the distribution characteristics of the dissolved fmercaptans between the tWo immiscible liquids, i. e. the alkali solution and the hydrocarbon oil. It has been customary to operate in a manner, best determined by experiment, which causes the greatest possible percentage of mercaptans to be removed in the aqueous alkali solution. Such methods of treating hydrocarbon oils accomplish with varying degrees of efficiency the desired end of eliminating the mercaptans as a group from the oil Where their presence is undesirable. The separation and recovery of individual mercaptans from such alkaline extracts, however, has been both difficult and costly.

We have discovered that repeated Washings of a mercaptan-containing hydrocarbon oil with controlled amounts of alkaline solution of controlled strength will produce alkali solutions containing varyng ratios of the individual mercap- 1938, Serial No. 203,094

tans, the rst Wash or Washes containing predominantly the lightest (lowest molecular weight) mercaptans which were originally contained in the hydrocarbon oil, and subsequent Washes containing successively heavier (higher molecular Weight) mercaptans.

It is therefore an object of our invention to provide a method whereby the various mercaptans contained in sour petroleum oils may be selectively removed and separately recovered inrelatively pure form, by properly controlling the distribution characteristics of an alkaline extracting medium in relation to the oil.

According to our process, the proper control of certain variable factors results in a controlled distribution of the successively higher molecular weight mercaptans between the oil and successive portions of alkaline solution with which the oil is extracted. The variable factors which must be properly controlled in order to achieve the improved selective separation vand recovery of finercaptans attained by our process are: (one) Proper choice of alkaline material, (two) concentration of the alkaline solution, and (three) the ratio of successive volumes of alkaline solution to the volume of oil to be extracted.

l'n carrying out the process of our invention to produce an alkaline extract which is rich in a particular mercaptan, it is necessary to choose washing conditions and to regulate the abovementioned variable factors in such a Way that the desired mercaptan will be concentrated in one phase or the other, While, at the same time holding at a minimum in this phase the concentration ci other mercaptans` present in4 the oil.

The alkaline material which We have found to be most effective in our process is sodium hydroxide. Cther alkaline materials may be used but, generally speaking, their use does not result in as high a selectivity under comparable conditions as is obtained with caustic soda. For example, potassium hydroxide which may be used intheprocess causes a higher degree of extraction than does caustic soda'under comparable conditions, but the selectivity with respect to the individual mercaptans is reduced.

The concentration of the alkaline solution may be varied depending on the results desired and the composition of the mercaptan mixture contained in the sour oil. Variation in alkali concentration will vary the proportions of the individual mercaptans extracted, the higher concentration favoring total mercaptan removal at the expense of selectivity, We have found that a relatively weak caustic soda solution having a gravity of about 10 B. gives a particularly eicient separation with the average mercaptan bearing stock when the relative volumes of successive wash solutions are properly chosen.

The temperature at which the extraction may be conducted is not critical. Generally speaking, a higher temperature will involve a readjustment of certain of the variable relationships mentioned above. Since the results attained are highly satisfactory, operation at ordinary atmospheric temperatures is usually preferred.

Having chosen a suitable alkali concentration, the number and relative volumes of the extraction treatment are then selected. This selection is governed by the composition of the mercaptan mixture present in the oil and by the degree of separation which is desired. When a high degree of separation is sought, it has been found desirable to use relatively small volumes of extraction solution in the iirst few treatments. In this way alkaline extracts are obtained by the irst washings which contain a higher proportion of low molecular weight mercaptans such as ethyl mercaptan, and a very low proportion of higher mercaptans. In other words, the mercaptan, removed by the small volume rst wash solution of a predetermined alkali concentration, is almost exclusively ethyl mercaptan. The next successive Washes will contain the remainder of the ethyl mercaptan not removed by the i'lrst wash and an ever increasing proportion of intermediate molecular weight mercaptans. The final wash will contain the hexyl and heavier mercaptans to the substantial exclusion of the lower molecular weight compounds. For convenience, we designate the volume of alkaline solution used in a particular step by its relation to the amount of oil extracted. Thus if 100 gallons of oil are to be extracted, a 1% wash involves the use of 1 gallon of alkaline solution, a 10% wash of 10 gallons, and a 100% wash of 100 gallons.

To determine the Various proportions of the diierent mercaptans which will be extracted from a mercaptan-bearing oil by a specific volume of alkaline solution of given concentration, and also to be able to calculate the approximate quantity of each mercaptan contained in the oil to be extracted, it has been found convenient to plot in the form of a graph certain experimentally determined Values. For example, the distribution characteristics of the individual mercaptans may be plotted from the values obtained by washing diierent solutions of individual mercaptans in pure hydrocarbons with various amounts of caustic soda solution. 'Ihe mercaptan content of the hydrocarbon phases after extraction, expressed in percentage of the original mercaptan concentration, is used as one coordinate. The other coordinate is conveniently what may be termed the caustic factor. The caustic factor is equal to 100-percent mercaptan remaining in the hydrocarbon percent of caustic wash used Curves representing each individual mercaptan together with curves for each different percentage wash are plotted. A sour oil is then subjected to a similar extraction treatment and a curve for this oil placed on the same chart. From the positions of the curves of the pure mercaptans and of the oil of unknown mercaptan composition, the approximate quantity of each mer- Original composition of mcrcaptan Composition of mercaptan mixture ex- Mercaptan mixture in sour tracted With one oil wash (1%) Percent Percent Hexyl and heavier... 30 0. 8

As a second example the sour oil was subjected to four alkali extractions with sodium hydroxide solution containing '70 g. NaOH per liter.

Each

extraction was a 100% wash, i. e., one Volume of lfresh alkali solution was used for each volume of oil at each wash.

Mercaptau Original composition of mercaptan mixture in sour oil Composition of mercaptan mixture'extractcd by alkali at each wash Wash Wash Wash Wash Percent 40 51.4 2.6 Trace lso-am 5 4. 6 9. 9 6.1 3. 1 Hexyl and heavier. 30 14,8 67.0 88.0 95. 5

Comparison of the rst example with the second indicates how the concentration of the individual mercaptans in the extract may be varied at will through the proper selection of conditions.

The following description supplemented by reference to the drawing constitutes a typical example illustrative of one advantageous method of carrying out the process of our invention. It is to be understood, however, that other types and forms of extraction and purifying equipment than those here illustrated in diagrammatic form may be employed in carrying out our process. Any other equipment may in fact be employed which affords an eicient means of obtaining intimate contact between the sour mercaptan-bearing hydrocarbon oil and the successive portions of alkali wash solution. As represented in the illustrative diagram; I is a sour oil feed line drawing the mercaptan-bearing oil from storage. 2 is a feed line drawing the alkali wash solution from storage. Valve 3 controls the flow of oil through pipe I, and valves 4, 5, and 6 control the flow of caustic through pipe 2. Vessels 1, 9 and II are mixing tanks and vessels 8, I'0, and I2 are settling tanks. Pumps I4, I6 and I8 transfer the oil from the settling tanks to the adjoining mixing tanks, and pumps I3, I5 and I'I transfer the oil-alkali solution mixture from the mixing tanks to the adjoining settling tanks through pipes 25, 26, and 2l controlled by valves 28, 29, and 30. Pipes 20, 22 and 24 controlled by valves I9, 2I and 23 are draw-off lines, respectively, for settlers`8, III and I2.

CII

In a preferred method of operation to separate mercaptans present` in one specific ratio, the sour hydrocarbon oil, such as raw gasoline, naphtha or kerosene is drawn through line I, controlled by valve 3 into mixer 'I. Caustic solution is drawn through line 2, controlled by valve 4 into mixer l. After mixing, pump I3 transfers the mixture through pipe 25 to settler 8. After settling, the caustic solution containing a high proportion of the lower mercaptans is drawn off to storage through pipe 2|] controlled by valve I9. The oil from settler 8 is withdrawn by pump I4 and discharged thru a pipe controlled by a Valve into mixer 9 where it is thoroughly mixed with a fresh proportion of caustic solution introduced from pipe 2 by valve 5. The mixture is withdrawn by pump I5 and conveyed through pipe 26 to settler Ill. A second portion of mercaptan containing caustic solution is drawn oi through pipe 22 controlled by valve 2I to storage, and the oil from this step is transferred by pump I6 to mixer II. Fresh caustic solution is again added from line 2 by valve 6 and after mixing, the material is withdrawn by pump I7 and transferred through pipe 2'I to settler I2. A third batch of mercaptan containing caustic is Withdrawn through pipe 2d controlled by valve 23 to storage, and the hydrocarbon oil is withdrawn from the settler by pump I8. Each successive portion of mercaptan-bearing caustic solution is then treated by methods known to the art, such as acid neutralization and/or steam distillation, for the recovery of the mercaptans.

The number of successive alkali washes and the volume of alkali introduced in each Wash is determined by the original content of mercaptans in the oil, and the degree of selective separation desired. In the above described illustrative example and in the diagram three alkali washes are indicated giving three alkaline extracts in which the various mercaptans have been preferentially extracted. Added units of mixing tanks and settling tanks are added to the circuit as necessary. For example, the original mercaptan content of the oil and the desired degree of separation may necessitate four or more successive extractions. Under such circumstances, one or more mixing and settling units with the necessary connections are added to the above-described circuit. It is also possible to use one container, for example, mixing tank l,V and treat a batch of oil by successively adding treating solution, mixing, settling, and removing the wash solution. This may be repeated as often as desired. The process is also adapted to the use of countercurrent operation by choosing the proper strength af alkali solution, and the proper ratio of alkali solution to hydrocarbonoil to be used for the extraction.

The individual mercaptans can be recovered from the caustic solution by known methods and further purified by known methods, such as for example, fractional distillation. In certain cases, particularly in the intermediate washings, the alkali extract may contain a mixture of mercaptans. It may be desired to recover one or more of the mercaptans contained therein, and this can be done by washing the alkali solution with controlled amounts of sweet, i. e., mercaptan-free hydrocarbon oil. The tendency is for the high molecular weight mercaptans to dissolve to a greater extent in the oil phase and the lower molecular weight mercaptans to remain in the alkali phase. Thus, by the proper sequence and/or combination of washing operations, any

individual mercaptan may be isolated in one phase or the other from which it may be extracted in a relatively pure state by known methods. It is also possible to obtain increased concentration of the lighter mercaptans in the alkali solution by washing successive batches of a raw sour hydrocarbon oil with the same alkali solution. For example if one or more further volumes of unextracted sour hydrocarbon oil are extracted with the first alkali wash solution containing the lowest molecular weight mercaptans, or subsequent alkali wash solutions containing the remainder of the loW molecular weight mercaptans, the concentration of the lighter mercaptans will be increased therein. An increased quantity of low molecular Weight mercaptan may thus be recovered from an alkali solution with a single recovery treatment.

We claim:

1. A process for the separation and recovery of individual mercaptans from a sour hydrocarbon oil which comprises selectively absorbing a low molecular Weight mercaptan in a volume of alkali solution small with respect to the volume of hydrocarbon oil, subsequently selectively absorbing successively higher molecular weight mercaptans in volumes of the alkali solution successively larger with respect to the volume of hydrocarbon oil, and recovering the thus separated mercaptans from the various portions of alkali solution.

2. A process for the separation and recovery of individual mercaptans from a sour hydrocarbon oil which comprises selectively absorbing a low molecular Weight mercaptan in a volume of sodium hydroxide solution small with respect to the volume of hydrocarbon oil, subsequently selectively absorbing successively higher molecular weight mercaptans in volumes of the sodium hydroxide solution successively larger with respect to the volume of hydrocarbon oil, and recovering the thus separated mercaptans from the various portions of sodium hydroxide solution.

3. A process for the separation and recovery of individual mercaptans from a sour hydrocarbon oil which comprises selectively absorbing a low molecular weight mercaptan in a volume of alkali solution small with respect to the volume of hydrocarbon oil, subsequently selectively absorbing successively higher molecular weight mercaptans in volumes of the alkali solution successively larger with respect to the volume of hydrocarbon oil, further enriching the said smaller volumes of alkali solution with respect to the lower molecular weight mercaptans by extracting therewith further volumes of unextracted sour hydrocarbon oil.

4. A process for the separation and recovery of individual mercaptans from a sour hydrocarbon oil which comprises selectively absorbing a low molecular weight mercaptan in a volume of alkali solution small with respect to the volume of hydrocarbon oil, subsequently selectively absorbing successively higher` molecular Weight mercaptans in volumes of the alkali solution successively larger with respect to the volume of hydrocarbon oil, decreasing the concentration of the higher molecular weight mercaptans in the intermediate portions of the said successively larger volumes of alkali solution by washing the alkali solution with controlled amounts of mercaptan free hydrocarbon oil, and recovering the thus separated mercaptans from the various portions of alkali solution.

5. In a process for the separation and recovery of individual mercaptans from a sour hydrocarbon oil, the improvement which comprises selectively absorbing a 10W molecular Weight mercaptan in a volume of alkali solution small With respect to the volume of hydrocarbon oil, subsequently selectively absorbing a higher molecular Weight mercaptan in a volume of alkali solution larger with respect to the volume of hydrocarbon oil, decreasing the concentration of higher molecular Weight mercaptans in the said smaller volume of alkali solution by Washing said solution with a controlled amount of mercaptan free hydrocarbon oil, and recovering the thus separated mercaptans from the respective alkali solutions.

WALLACE A. CRAIG. PAUL C. RICH. 

